1. Field of the Invention
The present invention relates to a hydrogen storage alloy which can electrochemically absorb and desorb hydrogen in a reversible manner, and to a nickel-metal hydride storage battery using the same.
2. Description of the Prior Art
In recent year, nickel-metal hydride storage batteries, which employ, as their negative electrodes, a hydrogen storage alloy which can electrochemically absorb and desorb hydrogen in a reversible manner, have been attracting attention in this art as rechargeable batteries having a high energy density and a long cycle life. Recent development in portable electric and electronic appliances utilizing the rechargeable batteries directed to high performance and wide diversification are drawing attention. Since the nickel-metal hydride storage batteries are excellent in their energy density and cycle life as compared with the conventional rechargeable batteries such as nickel-cadmium storage batteries, their production is estimated to increase in the near future.
The currently known hydrogen storage alloy is generally classified into two types: ABS-type mainly composed of a rare earth element and nickel (Ni) or the like metal, and AB.sub.2 -type mainly composed of zirconium (Zr), manganese (Mn) and the like metal. At present, nickel-metal hydride rechargeable batteries employing the hydrogen storage alloy of AB.sub.5 -type are mainly used as a power source of the portable electric and electronic appliances.
It has hitherto been known that the cycle life of the nickel-metal hydride storage battery using the hydrogen storage alloy of ABS-type can be improved by adding cobalt (Co) to the alloy. In view of this, a number of proposals have been made on the hydrogen storage alloy containing Co (for instance, Japanese Laid-Open Patent Publications No. Sho 60-250,558, No. Sho 62-139,258, No. Sho 63-164,161, No. Sho 63-304,570 and No. Hei 1-162,741). The currently available hydrogen storage alloys for the nickel-metal hydride storage batteries disclosed in these prior arts contain Co by 10 wt % or more.
Further, the hydrogen storage alloys of practical use at present have been still under development in order to make such alloys as homogeneous as possible, and in the case of AB.sub.5 -type alloy, the alloy microstructure has in general a macroscopically single phase. However, there are some proposals for making the alloy microstructure multi-phased and most proposals are directed to a co-existence of two hydrogen storage phases (for instance, Japanese Patent Publication No. Hei 6-38,333, Japanese Laid-Open Patent Publication No. Hei 5-195,122 and the like).
However, a storage battery having further longer cycle life is eagerly demanded in the commercial market. In particular, when an application for an electric vehicle (automobile) using a nickel-metal hydride storage battery as its energy source is developed, a storage battery having a longer cycle life which can repeat charging and discharging cycles for about 10 years will be required. And there has been a problem that the nickel-metal hydride storage batteries using the conventional hydrogen storage alloy do not have a sufficient cycle life.
Further, the alloys containing a large amount of Co demonstrate a favorable cycle life characteristic but have a disadvantage that their high-rate discharge characteristic of a battery configured with this alloy is inferior to that of the alloy containing no Co. In view of this, an achievement of a longer cycle life of the hydrogen storage alloy by any means except for the addition of Co has hitherto been desired.